1. Field of Invention
The invention relates to a mechanical jarring tool for insertion in and freeing a drill string or bit stuck in a well bore and particularly to an adjustable torsionally releasable latch or trigger therefor.
2. Description of the Prior Art
The drill bit or portions of the drill string can become stuck or jammed in the formation during drilling of a bore hole. Freeing the drill string can be achieved by means of impact or jarring devices which are carefully inserted in the drill string when formations tending to jam the drill string or bit are present. The impact or jar device is a telescoping tool which can be slid apart or contracted together comprising an arbor or mandrel and a sleeve or barrel with rigid stop means e.g. an anvil and hammer engageable at the end positions of both of the parts. It also comprises a releaseable latch or trigger arrangement which can fix the arbor and sleeve in a starting or neutral position. In this position spring energy can be produced and stored in the drill string by raising or lowering the drill string by means of the traction arrangement on the drill tower. Upon the application of sufficient upward or downward axial force the latch disengages and the stored spring energy is converted into kinetic energy, whereby the arbor and sleeve of the jar device are displaced until their contact or stop surfaces bump into each other. Because of the very large mass of the drill string which is thus suddenly braked, vigorous blows or jars which can loosen stuck parts are produced. Frequently, however, a number of blows are required until the drill string is again freed. The aforementioned latch or trigger arrangement has the duty of preventing longitudinal motion of the telescoping displacable parts until a preselected releasing axial force is impressed on the drill string, and after this force is exceeded, permitting the unhindered displacement of the arbor and sleeve according to the direction of the prestressing force. Portions of the latch arrangement are thereby exposed to forces as great as the releasing force. During continuous use, well jar parts stressed in this manner are subjected to special problems in regard to wear and risk of fracture. Therefore, construction must be planned so that tensile, pressure or shear forces lie as far as possible below the critical limit. A mechanical jarring tool of similar concept disclosed in U.S. Pat. No. 4,105,082 has parts of the latch arrangement, on which the releasing force impressed on the drill string is exerted, comprising an arbor and a sleeve which are mutually torsionally prestressed. In one place the sleeve is provided with studs which project into longitudinal slits or slots of the arbor; in another place the sleeve has an oblong opening or recess, on one side of which is an axially directed row of teeth of trapezoidal shape, which mesh with similar trapezoidal gear teeth of the arbor. When axial tension or compression is mutually imposed on the arbor and sleeve, the meshing teeth gear are pushed out of contact as a result of the simultaneous relative rotation of arbor and sleeve by the resultant force created by the axial force imposed upon the inclined faces of the teeth. Because arbor and sleeve are, however, fixed against relative rotation at another place by the studs, a torsional moment is built up which counteracts the separating tendency of the gear teeth.
The magnitude of the torsional moment which is found when the gear tooth segments move apart completely and the slope of the tooth profile define the releasing force. The torsional stressing of the arbor and sleeve for producing an interlocking force, which is in a direction opposed to the force which appears in the drill string, permits the avoiding of critical tensile, pressure or shear forces in the spring element as described in the U.S. patent. This is possible because the specific deformation path is small due to the large spatial expansion of the spring element. However, there are several characteristics which are detrimental to a successful application of the principle in the construction described in U.S. Pat. No. 4,105,082.
For example, after the arresting or latching device is exercized or released, the arbor and sleeve are still under torsional stress, which results in strong frictional forces between the engaging front surfaces of the gear tooth segments and the contact surfaces of studs and longitudinal splines. Besides severe wear of parts which slide over one another, the frictional forces also consume a portion of the kinetic energy of the detensioning drill string so that the intensity of the blow exercised during impact on the striking surface is diminished. Wear on the studs directed into longitudinal slots is especially pronounced due to the small contact surface and the consequently high surface pressure. Adjustment of the releasing force is possible only in coarse steps of increments and only with the help of a special tool.